The Vietnamese government is cracking down on itinerant SIM card users. A draft proposal by the Ministry of Information and Communications would limit individuals to three SIM cards per carrier; if they want an extra card, say for a data plan on a tablet or laptop, they'd need to buy one from another carrier. Since there are six carriers in Vietnam, this actually gives users a total of 18 cards at their disposal. Businesses, meanwhile, will be limited to 100 SIM cards per carrier, apparently to prevent them from reselling the cards on the open market.
According to VietNam News, this proposal is supposed to give regulatory agencies more reliable subscription data. It's also probably part of a continuing effort to stabilize a highly competitive phone industry. In 2009, the Ministry passed a law limiting subscribers to three phone numbers per carrier, likely in order to stop people from frequently buying new cards to take advantage of promotional deals. Eighteen SIM cards might be more than any one person would reasonably use, but the limit would still discourage subscribers from switching cards to get a particularly good discount.